In xquery, is there a difference between single and double quotes?

No. For string literals, they work exactly the same as in JavaScript.

Spec: XQuery 1.0 Section A.2.1: Terminal Characters - StringLiteral :

StringLiteral :: = 
    ('"' (PredefinedEntityRef | CharRef | EscapeQuot | [^" &]) * '"') | 
    ("'" (PredefinedEntityRef | CharRef | EscapeApos | [^' &]) * "'")